When a side collision occurs in which a vehicle collides with a side door of another vehicle, the side door deforms into the passenger compartment, and by the deformed side door, an occupant might be displaced inward of the passenger compartment from a position close to the side door. Also, depending on the height of the bumper of the vehicle that has collided into the side door, the lower section of the side door might deform into the passenger compartment more than the upper section of the side door. In this case, the lower section of the side door often deforms toward the lumbar region of the occupant.
Protection devices for protecting an occupant from a side collision as described above have been proposed in, for example, Patent Documents 1 and 2. The protection devices disclosed in Patent Documents 1, 2 protect an occupant from a side collision by inflating and deploying an airbag between the occupant seated in the rear seat and the side door of the vehicle. The airbag of such protection devices is inflated and deployed between the upper body of the occupant and the side door when a side collision occurs. The airbag inflated between the upper body of the occupant and the side door is displaced into the passenger compartment from a position close to the side door, accompanying the deformation of the side door into the passenger compartment caused by the side collision. Thus, the seated occupant is also displaced into the passenger compartment from the position close to the side door.
In general, the behavior of a vehicle during a side collision is a combination of parallel displacement and rotation. In particular, in a rotation of a vehicle, the position with respect to the center of gravity of the vehicle differs between the front side door and the rear side door. Thus, behavior of the vehicle in regard to rotation of the vehicle after collision differs between a side collision to the front side door and a side collision to the rear side door. Therefore, to secure occupant protection performance at the rear seat that is of the same level as that at the front seat during a side collision, occupant protection performance different from that of the front seat is desirably provided.
Patent Document 1: Japanese Laid-Open Patent Publication No. 8-40176
Patent Document 2: Japanese Laid-Open Patent Publication No. 8-310334